The present invention relates to an identification system composed of a proximity powered and coded portable unit and a corresponding energization and interrogation device which has a fixed installation. The portable unit may be in the form of a tag or card, and for convenience is referred to herein as a tag. However, the portable unit may also be incorporated in a badge, watch, ring or other article.
The present invention thus relates to a system wherein a fixed installation sends out energy to activate a responsive device which would ordinarily be carried by personnel and the device so energized would send out a coded signal to be picked up by a receiver which in turn would activate some system which, for example, functions to provide access to a controlled area, to keep track of the person, or to perform siimilar purposes.
In known systems of the above type power is provided by means of magnetic coupling, and the coded information is returned via the same path. Typical methods of encoding which have been proposed include switching of the Q of the coded tag receiver loop, by switching its frequency, introducing harmonics of the basic frequency, and other similar coding methods. These methods have proved difficult in practice because of the direct or harmonic relationship between the very large powering signal and the much weaker information signal.